This invention relates to a thick-film printed circuit board and a method for producing the same, the surface of which has a plurality of connecting conductor portions for enabling highly-accurate positioning of a passive or active element to be mounted on the printed circuit board.
In the conventional thick-film wiring plates to be connected with an electric element, a plurality of wiring conductors are laid on an insulating substrate and an end of each of the conductors constitutes a pedestal to be connected with a solder electrode of the electronic element.
A conventional thick-film printed circuit board will be briefly described below. A basic structure of connecting sections of a conventional thick-film printed circuit board for connection with an electronic element is shown in FIGS. 1a and 1b, which are respectively a plan view and a sectional view taken in line Ib--Ib in FIG. 1a, each illustrating the positional relation with a semiconductor chip making up an active electronic element. As seen from FIG. 1b, the semiconductor chip 4 is provided with ball-shaped solder electrodes 1. After the solder electrodes 1 are positioned on the pedestals 3 forming an essential part of the connecting sections of the thick-film printed circuit board 2 for connection with the electronic element, solder is remelted, thereby connecting the solder electrodes 1 and the pedestals 3 for connection with the electronic element 4. This process is called "solder reflow bonding" and finds practical application as a technique for connecting various elements to a thick-film printed circuit board in view of the availability of simultaneous connection of a multiplicity of connecting sections. In the thick-film printed circuit board using this technique, a construction is required in which solder electrodes 1 are prevented from flowing out and attaching to the other wires 6 at the time of remelting the solder. For this purpose, solder-blocking banks or dams 5 are formed surrounding the connecting pedestals for connection with the electronic element 4, thus securing the desired pedestal area.
In a conventional thick-film printed circuit boards produced by the screen printing process, the solder flow-blocking banks 5 and the wiring conductors 6 are formed by printing through different screens, and the resulting limitative conditions for manufacture pose the problem of a positioning error. Especially in recent years when more and more compact and detailed patterns of printed circuit boards are required, it has been difficult to achieve a sufficiently high dimensional accuracy of the pedestals, leading to a serious problem of a reduced yield. Ideally, as shown in FIG. 2a, substantially no positioning error occurs when the solder flow-blocking banks 5 are formed by screen printing. Actually, however, as shown in FIG. 2b, a positioning error occurs so that the pedestal 3a is reduced in width from w to w" and in length from d to d', while the pedestal 3b increases in width from w to w'. As a result, the pedestal 3a' is reduced in simple mechanical strength as compared with the pedestal 3a shown in FIG. 2a, while the pedestal 3b', is decreased in the connecting strength as compared with the pedestal 3b by the release of residual stress.
Further, while in the case of FIG. 2a, the solder poles are arranged substantially vertically, the solder poles in FIG. 2b have different heights and thicknesses, thereby presenting the danger of short-circuiting between adjacent solder poles. Also, the center distance between pedestals changes from P to P', probably displacing the connecting centers, with the result that residual strain attributable to the inclined fixing of the electronic element causes a reduced connecting strength due to thermal fatigue.